1. Field of the Invention
The invention relates to a process for the reduction of the polymer portion in crude diketene, which is produced by the catalytic pyrolysis of acetic acid and then the dimerization of the formed ketene.
2. Background Art
It is known that in the dimerization of ketene, besides the main product diketene, also resinous polymers and a ketene trimer are formed. [D. Borrmann in Houben-Weyl, Methoden der organischen Chemie (Methods of Organic Chemistry), vol. 7/4, 226, 228 (1968); and G. F. Pregaglia et al., Encyclop. Polymer. Sci. Techn., 8, 45 (1968)]. The formation of these ketene polymers, which are present in a portion of 8 to 10 percent in the crude diketene, means a loss of value in the dimerization of ketene and undesirable ballast or waste in the further processing.
Moreover, it is known that a portion of these polymers can be reduced by additives (inhibitors), such as, mineral acids, acetyl or benzoyl chloride, silicon tetrachloride, or sulfuryl chloride (German PS 700,218), or also by boron trifluoride, silicon tetrafluoride, tin(II) chloride, chloroacetic acid or p-toluene-sulfonic acid in the dimerization in a solvent (German PS 1,240,847). All of the above-cited known inhibitors are either protonic acids, metal or nonmetal halides or acid chlorides.
Another process for reduction of the polymer portion and for improvement of the storage stability of the diketene uses elementary sulfur in amounts of 0.1 to 2 percent by weight, relative to the amount of diketene (German PS 1,268,625). In this case, the sulfur is partially dissolved in the diketene but is mainly suspended therein.
But the inhibitors known so far exhibit a number of disadvantages. Thus, it has been shown that in the distillation of the diketene, metal compounds can even promote its polymerization, in other words, only shift the problem of resin formation. Also, for reasons of environmental protection, metal compounds as additives are undesirable, since in the end they must be eliminated as waste. On the other hand, acids and halogen compounds lead to corrosion of the apparatus and storage containers. The use of elemental sulfur has the disadvantage that, before the processing of the thus-stabilized diketene, a filtration and/or distillation is necessary for separation of the sulfur.